This section introduces aspects that may facilitate better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Nowadays, power converters are widely used in various fields. The power converters include many types. For example, based on their inputs and outputs, they can be classified into Direct Current to Direct Current (DC/DC) converters, Direct Current to Alternative Current (DC/AC) converters, Alternative Current to Direct Current (AC/DC) converters, and Alternative Current to Alternative Current (AC/AC) converters.
Particularly, DC/DC converters can be found in many application scenarios, for example, in data centers of communication systems. A DC/DC converter usually includes an input conversion circuit, such as a bridge circuit, an output rectifying circuit and an output filtering circuit. For an isolated DC/DC converter, it further includes a transformer connected between the input conversion circuit and the output rectifying circuit to provide isolation therebetween. Inductor-Inductor-Capacitor (LLC) topology is a serial resonant circuit and it was proposed for use in DC/DC converters mainly since it can achieve soft switching in the converters.
Due to space limitation and miniaturization requirements, power density of a power converter is constantly increasing and thus the current output thereof is increased substantially. In such a case, it is difficult to perform current sensing with traditional current sensing solutions. For example, one possible current sensing solution is to use a sampling resistor to sense the current. However, this solution has several drawbacks, such as low accuracy, large power loss, etc. At the same time, it requires additional temperature compensation since impendence of the sampling resistor is sensitive to temperature, which may influence the accuracy of current sensing. Another possible solution is to use a current transformer (CT) at the primary or the secondary of the converter to sense the current. For the purpose of illustration, FIG. 1 illustrates an example DC/DC converter with the LLC topology and a current sense structure in the existing solution.
As illustrated in FIG. 1, the example DC/DC converter 100 includes an input conversion circuit in a form of a bridge circuit, a transformer Tr with a primary and a secondary, an output rectifying circuit including diodes D1, D2, and an output filtering circuit including a capacitor Co, wherein the transformer Tr is connected between the input conversion circuit and the output rectifying circuit. The bridge circuit is formed by switches S1, S2, S3 and S4 connected in a full bridge topology. The DC/DC converter 100 further comprises a capacitor Cr and an inductor Lr, both of which are serially connected with the primary of the transformer Tr and thus form an LLC circuit together with a magnetizing inductor Lm of the primary. In order to sense the current, a current transformer CT is serially connected with the primary of the transformer Tr. The CT senses, by means of its primary Lsm serially connected with the primary, the current passing through the primary of the transformer Tr and generates a signal at its secondary Lss based on the current passing through the primary of the transformer Tr. The sensed current is integrated to obtain the integrated current, which is used to characterize the current passing through the secondary of the CT. However, this solution requires an additional sense transformer, which introduces more space, more power loss, more cost and at the same time it also involves issues such as low accuracy and load limitation. Moreover, due to the low accuracy, load waveform cannot be restored and in such a case, it is hard to achieve a cycle-by-cycle protection function, which however is desired in many power converter applications.
In addition, with the increasing of the power density of the power converter, another structure was also proposed for the power converter, which adopts parallel windings for the transformer. The use of parallel windings can decrease direct current resistance (DCR) so as to decrease winding loss and thus achieve better performance. Mostly, the parallel windings use the same via holes and are distributed to different Printed Circuit Board (PCB) layers, but sometimes the parallel windings are distributed in the same PCB layer or around different transformer legs. In such a case, it is hard to ensure exactly the same parallel windings and a mismatch among the windings may occur. Such mismatch may cause a power circulation among the parallel windings, which means an extra power loss. Moreover, the power circulation may lead to more voltage and current stress to power train devices. In addition, the mismatch may also increase the primary current and secondary root mean square (RMS) value and thus induce a conduction loss of a power switch, such as Metallic Oxide Semiconductor Field Effect Transistor (MOSFET), Insulated Gate Bipolar Transistor (IGBT), etc., and more winding loss of magnetic components, which may in turn generate more power dissipation of the power converter.
In an existing solution, it is proposed to use an additional capacitor in the power converter to reduce magnetic flux unbalance. For the purpose of illustration, FIG. 2 illustrates an example DC/DC converter with parallel windings in an existing solution. As illustrated in FIG. 2, the DC/DC converter 200 is similar to that as illustrated in FIG. 1 in most parts. Differences lie in that the output filtering circuit further includes an inductor Lo in addition to the capacitor Co, which is a typical output filtering circuit for hard switching, and that the DC/DC converter does not use the LLC topology but use parallel windings for the transformer Tr, while a capacitor Ca is serially connected with the primary of the transformer Tr. The use of the capacitor Ca can tackle the magnetic flux unbalance since it can eliminate accumulation of magnetic flux difference of the transformer, but it cannot eliminate the power circulation of parallel windings and problems associated therewith.